To unlock Kain, Judas and Marukka, beat the game with any of the original characters.

Hold down L1, L2, R1, R2, select, start, triangle, square, circle, and x during a fight.

To get an extra outfit for everyone but Marukka, Judas, and Cain, complete practice mode with the character you want the outfit for. After that, hold L1+L2 and press X to select your character.

To fight against the Gimp, complete practice mode with all eleven characters. After you have done this, hold L1+L2+R1+R2 and press X when selecting your opponent. This only works in practice mode.